Power control at a relay station in a wireless network

ABSTRACT

A wireless network includes a relay station (RS) for extending wireless coverage of a base station. Data is sent by the base station and relayed through the RS to a mobile station (MS), where the data is associated with a preamble that is sent directly from the base station to the MS. A transmit power of the RS is adjusted for transmitting the data from the RS to the MS to reduce a difference between a first power level of the preamble received at the MS and a second power level of the data received at the MS. The uplink transmit power of the MS for the data sent to the RS is adjusted to compensate for the difference in path loss from MS to base station and MS to RS and to compensate for the difference in noise_plus_interference level at RS compared to that of the base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/197,767, entitled “Power Control at a Relay Station in a WirelessNetwork,” filed Aug. 25, 2008 now U.S. Pat. No. 8,320,826, which claimsthe benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser.No. 60/957,831, entitled “Relay and Repeater Operation 5 Mechanisms,”filed Aug. 24, 2007, which is hereby incorporated by reference.

TECHNICAL FIELD

The invention relates generally to performing power control at a relaystation that is used for extending coverage of a base station in awireless network.

BACKGROUND

Various wireless access technologies have been proposed or implementedto enable mobile stations to perform communications with other mobilestations or with wired terminals coupled to wired networks. Examples ofwireless access technologies include GSM (Global System for Mobilecommunications) and UMTS (Universal Mobile Telecommunications System)technologies, defined by the Third Generation Partnership Project(3GPP); and CDMA 2000 (Code Division Multiple Access 2000) technologies,defined by 3GPP2.

Another type of wireless access technology is the WiMax (Worldwide.Interoperability for Microwave Access) technology. WiMax is based on theIEEE (Institute of Electrical and Electronics Engineers) 802.16Standard. The WiMax wireless access technology is designed to providewireless broadband access.

In a WiMax wireless network, base stations are provided to offerwireless access in corresponding coverage areas. An issue associatedwith using many base stations is that each base station requires adedicated backhaul connection in an access service network (for example,base stations are connected by these backhaul connections to one or moreASN gateways in a WiMax wireless network).

To alleviate the issue of having too many base stations andcorresponding dedicated backhaul connections, the concept of relaystations has been proposed for standardization in IEEE 802.16j, which isalso referred to currently as the Multihop Relay (MR) Specification.Within a coverage area of a base station (referred to as an MR basestation in the Multihop Relay context), one or more relay stations canbe provided to provide coverage for certain zones (which can be deadzones for the base station, for example) within the coverage area. Abenefit of using relay stations is that backhaul connections do not haveto be provided between the relay stations and other components of theaccess service network. A mobile station within the coverage area canconnect wirelessly with either the base station or a relay station inthe coverage area, for both uplink and downlink communications.

Information sent wirelessly on the downlink to a mobile station includesa frame start preamble and payload data. A preamble is sent from thebase station to the mobile station to allow the mobile station toacquire the downlink data (payload data). The preamble can includeinformation such as modulation scheme, transmission rate, length of timeto transmit an entire data frame, and so forth. In a Multihop Relaywireless network, a preamble may be sent directly from the base stationto the mobile station, even though the corresponding downlink data maybe relayed through the relay station to the mobile station. The preambleis sent by the base station to the mobile station is typically highlycoded and thus well protected from interference (to increase thelikelihood of reliable receipt by the mobile station in a coveragearea). On the other hand, downlink data sent from the base station isnot as highly coded and therefore not as protected from interference. Asa result, in certain zones within a coverage area, downlink data anduplink data would have to be relayed through a relay station rather thancommunicated directly from the base station to the mobile station.

When the mobile station is relatively far away from the base stationwithin a coverage area, the preamble received by the mobile station fromthe base station suffers a relatively large path loss from the basestation to the mobile station. However, even though the mobile stationmay be far away from the base station, the mobile station can be quiteclose to a relay station that is relaying downlink data to the mobilestation. As a result, the downlink data received from the relay stationby a mobile station can have a power level that is significantly higherthan the power level of the preamble received by the mobile station.

A wireless receiver in a mobile station typically includes an adjustablegain control (AGC) circuit that is used to adjust a gain applied to areceived signal based on some indication of the expected power level ofreceived signals. If the automatic gain control is performed based onthe power level of the preamble received directly from the base station,then this may cause received downlink data that is at a high power levelfrom the relay station to be out of range, which can cause the AGCcircuit to not be able to receive the downlink data properly. Morespecifically, the AGC circuit in the mobile station may not be able totrack the sharp variation of power of the downlink data as compared tothe preamble power, which can cause a data recovery failure. Moreover,open loop power control for uplink transmission, which is performedusing downlink path loss and/or uplink noise plus interference levelinformation, would not operate as desired, which can cause enhanceduplink interference. With a relay station present, the downlink pathloss is from the relay station to the mobile station instead of the basestation to the mobile station; similarly the uplink noise andinterference should be measured at the relay station instead, of thebase station.

SUMMARY

In general, according to an embodiment, a method for use in a wirelessnetwork including a base station and a relay station for extendingwireless coverage of the base station includes communicating downlinkdata sent by the base station and relayed through the relay station to amobile station, wherein the downlink data is associated with a preamblethat is sent directly from the base station to the mobile station. Atransmit power of the relay station is adjusted for transmitting thedownlink data from the relay station to the mobile station to reduce adifference between a first power level of the preamble received at themobile station and a second power level of the downlink data received atthe mobile station.

In another embodiment, the uplink transmit power of the mobile stationis adjusted so as to take into account the difference between the basestation-to-mobile station path loss and the relay station-to-mobilestation path loss, and the fact that the noise plus interference levelis measured at the relay station instead of the base station.

Other or alternative features will become apparent from the followingdescription, from the drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary wireless network thatincorporates an embodiment of the invention.

FIG. 2 is a graph that illustrates various power levels associated withcommunications among a base station, relay station, and mobile stationsin the wireless network of FIG. 1.

FIG. 3 illustrates exemplary coverage zones of relay stations in acoverage area of a base station.

FIG. 4A is a message flow diagram of a process of adjusting a powerlevel for downlink communication from the relay station, in accordance,with an embodiment.

FIG. 4B is a message flow diagram of a process of adjusting a powerlevel of uplink communication from the mobile station to the relaystation, in accordance with an embodiment.

FIG. 5 is a graph that illustrates multiple mobile station groups andcorresponding relay station transmit powers, in accordance with anotherembodiment.

FIG. 6 is a flow diagram of a process of grouping mobile stations intomultiple groups, in accordance with another embodiment.

FIG. 7 is a block diagram of a system such as a base station or relaystation, in which an embodiment can be incorporated.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of some embodiments. However, it will be understood bythose skilled in the art that some embodiments may be practiced withoutthese details and that numerous variations or modifications from thedescribed embodiments may be possible.

FIG. 1 illustrates a portion of a wireless network, such as a WiMax(Worldwide Interoperability for Microwave Access) network, or other typeof network, that includes a base station 100, a relay station 102, andmobile station 104. Note that although reference is made to WiMax andstandards corresponding to WiMax, it is noted that techniques accordingto some embodiments are applicable to other types of wireless networksin which mobile stations can receive control information from the basestation and data from the relay station. Other possible wirelesstechnologies include the LTE (Long Term Evolution) technology, from3GPP, which seeks to enhance the UMTS (Universal MobileTelecommunications System) technology; code division multiple access(CDMA) technology; orthogonal frequency division multiple access (OFDMA)technology; and so forth. In some embodiments, the base station, relaystation, and mobile station operate according to the Multihop Relay (MR)Specification as defined in a proposed IEEE 802.16j standard. Note thatalthough reference is made to the MR Specification and the IEEE 802.16jstandard, it is noted that embodiments of the invention are applicableto the standard as it is modified or evolved over time, as well as tosubsequent standards that evolve from the MR Specification and the IEEE802.16j standard. The Multihop Relay context, the base station 100 isreferred to as an MR base station.

The mobile station 104 of FIG. 1 is within a coverage area of the basestation 100. The relay station 102 in the coverage area of the basestation 100 is provided to extend the wireless coverage of the basestation 100 for a particular zone of the coverage area that maycorrespond to a zone of reduced coverage (e.g., dead zone) of the basestation 100. A “relay station” refers to any component in the coveragearea of a base station that is able to receive information from the basestation (in the downlink direction), and forward the information to themobile station. In the uplink direction, a relay station is a componentthat receives information from the mobile station, and forwards thatinformation from the relay station to the base station. In someimplementations, the relay station has digital processing capabilities,and is able to extract information, decode/encode the information,demodulate/modulate the information, and so forth.

FIG. 1 illustrates the communication of downlink information from thebase station 100 to the mobile station 104 that is within zonecorresponding to the relay station 102. The downlink informationincludes a preamble (or other type of control information) that is sentdirectly from the base station 100 to the mobile station 104 along path106. The downlink information further includes downlink data (that isassociated with the preamble) that is sent along path 108 from the basestation 100 to the relay station 102. In turn, the relay station 102relays the downlink data along path 110 from the relay station 102 tothe mobile station 104. In the ensuing discussion, reference is made topreambles; note that techniques according to some embodiments are alsoapplicable to downlink transmissions in which downlink data isassociated with other types of control information.

Various parameters are also represented in FIG. 1. Pbs represents thepower of the preamble transmitted by the base station 100 to the mobilestation 104. Lbs represents the path loss (in terms of power) along path106 from the base station 100 to mobile station 104. Ppr (which is equalto Pbs−Lbs) is the power of the preamble as received by the mobilestation 104.

Prs represents the power of the downlink data as transmitted by therelay station 102, while Lrs represents the path loss along path 110from the relay station to the mobile station. Pdata represents the powerof the downlink data as received by the mobile station 104.

Note that if the mobile station 104 were not in the zone of the relaystation 102, then both the preamble and downlink data would be sentdirectly from the base station 100 to the mobile station 104.

Note also that the mobile station can send uplink data (not shown)either directly to the base station or relayed to die base stationthrough the relay station.

When the mobile station 104 is relatively far away from the base station100, the preamble received by the mobile station from the base stationsuffers a relatively large path loss from the base station to the mobilestation. However, even though the mobile station may be far away fromthe base station, the mobile station can be quite close to the relaystation 102 that is relaying downlink data to the mobile station. As aresult, the downlink data received from the relay station by a mobilestation can have a power level (Pdata) that is significantly higher thanthe power level of the preamble (Ppr) received by the mobile station.

A wireless receiver in the mobile station 104 typically includes anadjustable gain control (AGC) circuit that is used to adjust a gainapplied to a received signal based on some indication of the expectedpower level of received signals. If the automatic gain control isperformed based on the power level of the preamble received directlyfrom the base station 100, then this may cause received downlink datathat is at a high power level from the relay station to be out of range,which can cause the AGC circuit to not be able to receive the downlinkdata properly.

In accordance with some embodiments, to address this issue, a mechanismis provided to enable the power level (Prs) of the relay station 102 tobe adjusted (e.g., reduced) to address the issue associated with thedownlink data power (Pdata) being significantly higher than the preamblepower (Ppr).

In another embodiment, it is noted that the uplink transmission power ofthe mobile station also should be adjusted based upon the differencebetween the base station-to-mobile station path loss and the relaystation-to-mobile station path loss, and to take into account the factthat the noise plus interference level is measured at the relay stationinstead of the base station.

FIG. 2 graphically depicts adjustment of the relay station transmit,power (Prs) for transmitting downlink data from the relay station to themobile station. The original relay station transmit power Prs (prior toadjustment) is represented as Prs(original) in FIG. 2, whereas theadjusted relay station transmit power Prs is represented as Prs(new) inFIG. 2. Note that in the graph of FIG. 2, the left hand side of thegraph represents higher power while the right hand side of the graphrepresents lower power.

Positions of the base station, relay station, and a cell edge arerepresented with vertical dashed lines in FIG. 2. A vertical dashed line202 represents the base station position, a vertical dashed line 204represents the relay station position, and the vertical dashed line 206represents the cell edge position (the edge of the coverage area of thebase station).

An arrow 208 from the base station position 202 to the cell edgeposition 206 represents the maximum path loss of information transmittedby the base station to a mobile station located at the cell edgeposition 206.

An arrow 210 from the base station position 202 to the relay stationposition 204 represents the minimum path loss (Lbs in FIG. 1) for thepreamble sent by a base station to mobile stations. What this means isthat for a mobile station that is located at the relay station position204, the path loss experienced for the preamble sent by the base stationis the path loss along path 108 (FIG. 1) between the base station and arelay station.

Double arrow 212 represents the range of the power level Ppr of thepreamble that can be received by the mobile stations within the coveragezone of the relay station. Ppr varies depending on how close or how faraway a mobile station is from the base station. Note that Ppr=Pbs−Lbs(transmit power of base station less the path loss Lbs from the basestation to the mobile station).

In FIG. 2, assuming that the transmit power of downlink data from therelay station is Prs(original), then the possible range of the power ofthe downlink data (Pdata) as received by mobile stations is indicated by214 in FIG. 2. The Pdata range specifies the possible Pdata power levelsthat can be observed by different mobile stations depending on theirpositions relative to the relay station. Within the range 214, a firstsubrange 216 specifies a region in which Pdata is greater than thelargest Ppr value that can be observed by any mobile station, and asecond subrange 218 specifies a region in which Pdata is less than thelargest Ppr value. For a mobile station that receives downlink data at apower level within the first subrange 216, there is some likelihood thatPdata will be greater than Ppr by a sufficient amount such that the AGCcircuit of the mobile station receiver will not be able to correctlyrecover the received downlink data.

To address the above issue, the transmit power Prs of the relay stationis adjusted such that it becomes lower, represented as Prs(new) in FIG.2. With the reduced Prs value, the Pdata range is represented as 220 inFIG. 2. With the reduced transmission power of the relay station, it ismore likely that the downlink data will be received at a power that willbe correctly recovered by the mobile station, since Pdata will likely beless than Ppr, or will exceed. Ppr by a small enough margin that the AGCcircuit in the mobile station receiver will still process the downlinkdata properly.

The power reduction of Prs for the relay station can be accomplishedwithout adversely impacting the C/I (carrier-to-interference ratio)performance for downlink data sent from the relay station to the mobilestation if the relay station and base station operate in two differentchannels (in other words, there is frequency and/or time separationbetween the relay station and base station), and either one of thefollowing two conditions are provided.

First, the system is noise limited in that coverage zones of relaystations are sufficiently far apart such that a relay station does notexperience interference from outside the coverage zone of the relaystation, such as interference from another relay station. Since nointerference is present, relay station transmit power Prs can be reducedto a minimum (some predefined decibel (dB) above the noise level).

Second, in a system that is interference limited (where a mobiletransmission to/from the relay station experiences interference fromother relay stations), the following plan (depicted in FIG. 3) can beimplemented to manage relay station-to-relay station interference.

FIG. 3 illustrates deployment of multiple relay stations 102 within acoverage area of the base station 100. The respective coverage zones 302for corresponding relay stations 102 are depicted as white regions.Within these regions 302, the C/I margin is a predefined dB above thenoise level, which means that adequate coverage is provided by thecorresponding relay station 102 in the coverage zone 302.

Outside of each coverage zone 302 of a relay station 102 is acorresponding region 304 (cross-hatched) that cannot be covered by thecorresponding relay station 102, due to weak C/I in the region 304. Inthese regions 304, the base station 100 is used to provide coverage. Byplanning the coverage zones 302 of multiple relay stations 102 within abase station coverage area properly, reduction of transmit powers Prs ofthe relay stations does not prevent adequate performance fortransmission of downlink data by the relay stations 102.

It is noted that the downlink transmit power Prs can be adjusted eitherby the base station or the relay station. In the embodiment in which thePrs adjustment is controlled by the base station, the followingprocedure can be performed, as depicted in FIG. 4A. When the mobilestation first accesses a wireless network, the mobile station may send(at 450) ranging information (e.g., a ranging code) to the base station.An example of the ranging procedure is described in the IEEE 802.16estandard. This ranging information is received by both the base stationand the relay station. In response to the ranging information, the relaystation forwards (at 452) a message to the base station, with themessage containing information (e.g., power level) from which the basestation can determine the path loss from the relay station to the mobilestation. Based on the ranging information received from the mobilestation and the message received from the relay station, the basestation is able to determine (at 454) the difference between a firstpath loss from the base station to the mobile station and a second pathloss from the relay station to the mobile station. Based on thedifference between the first and second path losses, the base stationsends (at 456) a control message to the relay station to adjust Prs.Note that adjustment of Prs can also compensate for differences inantenna gain, differences in receiver noise, differences in cablelosses, and so forth.

Alternatively, instead of the procedure of FIG. 4A, another technique ofadjusting Prs involves the mobile station measuring a preamble signalpower level that is transmitted by the base station. The measurement ofthe preamble signal power level is then sent in the uplink to the basestation (directly or via the relay station), from which the base stationcan derive the base station-to-mobile station path loss.

For a TDD system, the RS-MS path loss can be deduced from uplinkmeasurement at the Relay station. Alternatively, MS can be instructed tomeasure a data or beacon signal from the RS. In response to the measuredpower level of the signal from the relay station to the mobile station,the mobile station sends the measurement data back to the base station,from which the base station can derive the relay station-to-mobilestation path loss.

The above describes techniques for adjusting relay station downlink Prsfor a time division duplex (TDD) system (in which the same frequency isused for uplink and downlink communications between the relay stationand mobile station). Similar techniques can also be applied to frequencydivision duplex (FDD) systems. In an FDD system, uplink measurementscannot be used for deducing downlink path loss. In the FDD system, todetermine downlink path losses, the mobile station can measure signalsfrom the base station and relay station, and then send thosemeasurements to the base station. For the uplink path losses, the mobilestation can use, for example, ranging signals sent by the mobilestation, with the relay station and base station observing the receivedlevel of such ranging signal. The relay station will forward itsmeasurement to the base station to allow the base station to determinethe difference between the base station path loss and relay station pathloss.

As noted above, in another embodiment, the uplink transmit power of themobile station is adjusted based upon the difference between the basestation-to-mobile station path loss and the relay station-to-mobilestation path loss, and taking into account the fact that the noise plusinterference level is measured at the relay station instead of the basestation.

In accordance with some embodiments, FIG. 4B shows a procedure foradjusting the uplink transmission power of the mobile station for uplinktransmission to the relay station to compensate for the difference inuplink path losses (between base station and mobile station and betweenrelay station and mobile station). The relay station measures the uplinktransmission (sent at 402) of the mobile station. Based on themeasurement of the uplink transmission, the relay station sends (at 404)to the base station an indication of the power level of the uplinktransmission measured by the relay station. In addition, the relaystation informs the base station of the noise plus interference (N+I)level experienced by the relay station in the uplink.

Based on these two measurements, the base station can calculate at 406)the path loss difference as well as the N+I difference, and an adjustedN+I is then sent (at 408) from the base station to the mobile station.In some systems, such as those based on the IEEE 802.16e standard, theN+I information is broadcast and cannot be specified for an individualmobile station. Therefore, the adjustment may be included as a mobilestation specific parameter (e.g., the Offset_SSperSS_and/orOffset_BSperSS parameters according to IEEE 802.16e, which can beadjusted to reflect the differences).

In addition, for a TDD system, these uplink measurements can be used toestimate the path loss differences in the downlink and the base stationcan send the relay station a signal to adjust the relay station downlinktransmit power as required to avoid or reduce the likelihood ofexcessive Pdata causing data recovery failure at the mobile station.

The mobile station then modifies (at 410) the power of its uplinktransmission to the relay station based on the adjusted NI. The uplinktransmission according to the modified power is sent (at 412) by themobile station to the relay station.

It is noted that the procedure of FIG. 4B can also be extended toperform downlink power control of Prs. The relay station is aware thatthe power level of the uplink transmission (412) is to have beenmodified by some incremental amount and therefore can continuallymonitor path loss difference Lbs−Lrs. In this way, the uplinktransmission will not suffer from not taking into account that the N+Inoise plus interference level) measurement at the relay station isdifferent from the same measurement at the base station. For a TDDsystem, the adjustment of the downlink transmission power Prs based onthe measured uplink transmission (412) can be performed since the samefrequency is used for both the uplink and downlink transmissions betweenthe relay station and the mobile station. For a TDD system, since thesame frequency is used for both the uplink and downlink, it is expectedthat the path loss between the relay station and mobile station in boththe uplink and downlink directions is the same.

For an FDD system, the relay station downlink power can be adjusted bythe mobile station measuring the signals from both the base station andrelay station, and then sending those measurements to the base station.For the uplink, the mobile station power adjustment can be done, forexample, by using the ranging signal sent by the mobile station andobserving the received power level of the ranging signal at the basestation and relay station. The relay station forwards its measurement tothe base station, and the base station instructs the mobile station toperform the adjustment that takes into account the path loss differenceof the base station-mobile station path and the relay station-mobilestation path in the uplink and the noise plus interference level.

In an alternative embodiment, instead of the relay station making thedecision regarding how much to adjust Prs, the base station can insteaddetermine how much adjustment of Prs is necessary. The base station canthen send the notification of the amount of adjustment of Prs needed tothe relay station.

The embodiments described above are applicable to relay stations withrelatively small coverage zones. However, for relay stations havingrelatively large coverage zones (where a relay station is considered tohave a large coverage zone if the relay station transmit power is greatenough to cause interference at another relay station), reduction ofrelay station transmit power Prs in the manner described above cannotresolve the AGC problem. Deploying relay stations in the mannerdescribed above result in an interference limited system wheresubstantial interference among relay stations can prevent properoperation. In such a scenario, unless all the relay stations reducepower at the same time, the C/I performance of some relay stations maybe adversely impacted.

To address the above issue, mobile stations served by any relay stationare grouped into multiple mobile station groups based on their path lossdifferences (Lbs−Lrs difference). Recall that Lbs is the path loss fromthe base station to the mobile station, and Lrs is the path loss fromthe relay station to the mobile station. Depending on the path lossdifference, the transmit power level of the relay station is to bereduced to substantially equalize the base station and the relay stationtransmit power levels (the power levels do not have to be exactlymatched, but they should be equilibrated to the point to obtain atractable level difference). The mobile station groups with similardifferences in the path loss are served at the same time by all therelay stations.

In the example of FIG. 5, three mobile station groups MSG1, MSG2, andMSG3 are defined. For mobile station group MSG1, the relay station poweris reduced to Prs1. For mobile station group MSG2, the relay stationpower is reduced to Prs2, where Prs2 is less than Prs1. For mobilestation group MSG3, the relay station power is reduced to Prs3, wherePrs3 is less than Prs2.

FIG. 5 also depicts a Pdata range 502 for an original relay stationtransmit power Prs. However, for MSG1, where the relay station transmitpower is Prs1, the Pdata range 502 has been reduced to range 504. Notethat the Pdata range 504 for MSG1 is in the lower portion of the range520 for Ppr (the preamble power received by the mobile station) asdepicted in FIG. 5.

The Pdata range for the second mobile station group, MSG2, isrepresented as 506 (which corresponds to the relay station transmitpower of Prs2), and the Pdata range for the third mobile station group,MSG3, is represented as 508 (which corresponds to the relay stationtransmit power of Prs3).

The different mobile station groups MSG1, MSG2, and MSG3 are served bythe relay station at different times. In other words, within a firsttime interval, the relay station transmits downlink data to mobilestations in group MSG1 at power Prs1; within a second time interval, therelay station transmits downlink data to mobile stations in group MSG2at power Prs2; and within a third time interval, the relay stationtransmits downlink data to mobile stations in group MSG3 at power Prs3.

Note that all relay stations within a coverage area of a base stationare set to transmit at the same power at the same time. In other words,all relay stations within the coverage area will transmit at Prs1 in thefirst time interval; all relay stations within the coverage area willtransmit at Prs2 in the second time interval; and so forth. This ensuresthat the C/I performance is not adversely impacted.

Note that it is possible that some relay stations may have fewer mobilestation groups (due to fewer mobile stations being in the correspondingcoverage zones or fewer path loss differences).

FIG. 6 illustrates a procedure for performing downlink communicationswith mobile stations in multiple mobile station groups, as discussedabove. For each mobile station in the coverage zone of a relay station,the relay station can measure (at 602) the uplink path loss from themobile station to the relay station. The uplink path loss is equal tothe downlink path loss Lrs, due to reciprocity (the same frequency isused on the uplink and downlink between the relay station and mobilestation). The relay station then determines (at 604) the path lossdifference (Lbs−Lrs) for each mobile station in the relay station'scoverage zone. Based on the calculated path loss differences, the relaystation next groups (at 606) the mobile stations into corresponding onesof multiple mobile station groups. For example, if the path lossdifference has a range between Lmin and Lmax (minimum path loss andmaximum path loss, respectively), then the MSG3 group can be definedfrom Lmin to L1 the MSG2 group can be defined from L1 to L2, and theMSG1 group can be defined from L2 to Lmax, where Lmin<L1<L2<Lmax. Thenumber of mobile station groups are selected based on how muchdifference between the preamble receive power (Ppr) and the data receivepower (Pdata) the AGC circuit of a mobile station can tolerate.

Since a mobile station may move within a coverage area, the uplink pathloss of each mobile station must be continually monitored and somereassignments in power levels and group assignments may have to beperformed.

The relay station next assigns (at 608) respective transmit power levelsPrs1, Prs2, Prs3) to the corresponding mobile station groups (e.g.,MSG1, MSG2, MSG3). Downlink communication can then proceed between therelay station and corresponding mobile stations in the relay station'scoverage zone using one of the plural transmit powers (e.g., Prs1, Prs2,Prs3) based on which group each mobile station is located in.

FIG. 7 is a block diagram of a system 700, which can be one of the basestation 100 or relay station 102 (FIG. 1). The system 700 includessoftware 702 executable on one or more central processing units (CPUs)704 that is (are) connected to a storage 706. The software 702 isexecutable to perform tasks described above. The system 700 furtherincludes a wireless interface 708 connected to an antenna 710 to performwireless communications (e.g., radio frequency or RF communications).

Instructions of the software 702 are loaded for execution on aprocessor, such as the one or more CPUs 704. The processor includesmicroprocessors, microcontrollers, processor modules or subsystems(including one or more microprocessors or microcontrollers), or othercontrol or computing devices. A “processor” can refer to a singlecomponent or to plural components.

Data and instructions (of the software) are stored in respective storagedevices, which are implemented as one or more computer-readable orcomputer-usable storage media. The storage media include different formsof memory including semiconductor memory devices such as dynamic orstatic random access memories (DRAMs or SRAMs), erasable andprogrammable read-only memories (EPROMs), electrically erasable andprogrammable read-only memories (EEPROMs) and flash memories; magneticdisks such as fixed, floppy and removable disks; other magnetic mediaincluding tape; and optical media such as compact disks (CDs) or digitalvideo disks (DVDs).

In the foregoing description, numerous details are set forth to providean understanding of the present invention. However, it will beunderstood by those skilled in the art that the present invention may bepracticed without these details. While the invention has been disclosedwith respect to a limited number of embodiments, those skilled in theart will appreciate numerous modifications and variations therefrom, itis intended that the appended claims cover such modifications andvariations as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A method for use in a wireless network includinga base station and a relay station for extending wireless coverage ofthe base station, the method comprising: adjusting a transmit power of arelay station for transmitting downlink data, received from a basestation, from the relay station to a mobile station to reduce adifference between (1) a first power level of control informationreceived directly from the base station at the mobile station, and (2) asecond power level of the downlink data received from the relay stationat the mobile station.
 2. The method of claim 1, wherein adjusting thetransmit power is performed by the relay station.
 3. The method of claim1, wherein adjusting the transmit power is performed by the basestation.
 4. The method of claim 1, wherein the transmit power of therelay station is adjusted based upon a message from the base station. 5.The method of claim 4, further comprising the base station determining adifference between a first path loss from the base station to the mobilestation and a second path loss from the relay station to the mobilestation, wherein the adjustment of the transmit power of the relaystation is based upon the determined difference.
 6. The method of claim1, further comprising: dividing plural mobile stations within a zone ofthe relay station into multiple groups; and communicating with themobile stations in the multiple groups at different times.
 7. The methodof claim 6, further comprising: transmitting, by the relay station,downlink data to the mobile stations in the different groups atdifferent transmit powers.
 8. The method of claim 7, whereintransmitting the downlink data to the mobile stations in the differentgroups at different transmit powers comprises at least: transmittingdownlink data from the relay station to a mobile station in a firstgroup at a first transmit power; and transmitting downlink data from therelay station to a mobile station in a second group at a second transmitpower.
 9. The method of claim 7, further comprising: storing informationrelating to multiple groups of mobile stations, wherein mobile stationsare assigned to the multiple groups based upon relay station path lossesbetween the mobile stations and the relay station.
 10. The method ofclaim 9, wherein base station path losses are present between the basestation and the mobile stations, and wherein the mobile stations areassigned to the multiple groups based upon differences betweenrespective base station path losses and respective relay station pathlosses.
 11. A base station for use in a wireless network having a relaystation that is able to relay data between a mobile station and the basestation, the base station comprising: a processor configured to adjust atransmit power of a relay station for transmitting downlink datareceived from a base station to achieve a relationship between (1) afirst power level of control information received by a mobile stationdirectly from the base station, and (2) a second power level of thedownlink data received by the mobile station from the relay station suchthat the mobile station is able to successfully recover the downlinkdata.
 12. The base station of claim 11, wherein the processor isconfigured to assign mobile stations to multiple groups of mobilestations based upon relay station path losses between the mobilestations and the relay station.
 13. The base station of claim 12,wherein base station path losses are present between the base stationand the mobile stations, and wherein the mobile stations are assigned tothe multiple groups based upon differences between respective basestation path losses and respective relay station path losses.
 14. Therelay station of claim 11, wherein the processor is further configuredto determine a difference between a first path loss between the basestation and the mobile station and a second path loss between the relaystation and the mobile station, wherein the transmit power of the relaystation is adjusted based upon the difference.
 15. The relay station ofclaim 11, wherein the base station is a Worldwide Interoperability forMicrowave Access (WiMax) base station.
 16. A base station fortransmitting data to a mobile station via a relay station, the basestation configured to: transmit downlink data to a mobile stationthrough a relay station; and adjust a transmit power of the relaystation for transmitting the downlink data from the relay station to themobile station to reduce a difference between (1) a first power level ofcontrol information received at the mobile station from a base station,and (2) a second power level of the downlink data received at the mobilestation from the relay station.
 17. The base station of claim 16,wherein base station is configured to: group mobile stations intomultiple groups according to path losses between the mobile stations andthe relay station.
 18. The base station of claim 17, wherein the relaystation transmits downlink data to the mobile stations in different onesof the multiple groups at different power levels.